1. Field of the Invention
The present invention relates to a method for making over-indexed thermoplastic polyurethane elastomer precursor. The invention also relates to thermoplastic polyurethane elastomer made from the precursor.
2. Description of Related Art
Polyurethane is a polymer widely used in many industries. One feature of polyurethane is that it can be made in thermoplastic form or thermoset form. Thermoplastic polymer materials may be reversibly melted, and so may be used in a variety of manufacturing techniques, such as compression molding, that take advantage of this property. On the other hand, thermoset polymer materials are generally formed by mixing two or more components to form a cured polymer material that cannot be re-melted or re-worked. Each type of polymer material present advantages and disadvantages when used to manufacture various articles, including golf balls.
Because polyurethane typically is the reaction product of an isocyanate moiety and a hydroxyl moiety, the properties and characteristics of polyurethane can be adjusted to an extent by selecting specific reactants. For example, both long-chain polyols and short polyols often are used in the same polyurethane molecule. Long-chain polyols provide soft areas or segments in the polyurethane and short-chain polyols, in combination with isocyanate, form hard areas or segments in the polyurethane. However, the identity and molecular weight of reactants are not the only determinants of the properties and characteristics of the polyurethane product, as can be seen from the differences between thermoset and thermoplastic forms of polyurethane.
In golf ball technology, many attempts have been made to use thermoset polyurethane elastomers as a substitute for early (balata) and late (ionomers) compositions for making, for example, outer cover layers. Ionomers often are softened to improve feel and control. Thermoset polyurethane elastomers are relatively inexpensive and offer good hitting feel and good scuff resistance. Particularly, thermoset polyurethane elastomers may present improvements in the scuff resistance as compared to softened ionomer resin blends. However, thermoset materials require complex manufacturing processes to introduce the raw material and then effect a curing reaction, which causes the manufacturing process to be less efficient.
Although thermoset polyurethane can be made into tough, damage-resistant articles, these articles are not easily recycled. Thermoplastic polyurethane is softened by heat and therefore is amenable to recycling. Thermoplastic polyurethane often is made by preparing a precursor, and typically is cured in stages during various processing steps. However, thermoplastic polyurethane typically is less durable and less scuff-resistant than thermoset polyurethane.
Whereas it is possible to proceed in continuous processing steps from reactants (isocyanate and hydroxyl, for example) to finished product, such processing typically is difficult to carry out successfully and efficiently. Such continuous processing typically requires coordination of many different processes and often requires integration of another product, such as a product to be covered by the polyurethane, into the manufacturing processing. Such coordination is difficult. Thus, a typical operating technique is to manufacture thermoplastic polyurethane separately, and then to use this pre-made product to form the article. In this way, processing is segmented and provides easier management of each segment.
Additional challenges are introduced when the thermoplastic polyurethane is over-indexed. Manufacture and storage of over-indexed polyurethane is typical, but free isocyanate moieties in the product are known to be hygroscopic and to react in the presence of water. Therefore, current processing technique for such stored thermoplastic polyurethane requires drying before use. Even so, this process has been found wanting, and the resultant thermoplastic polyurethane elastomer may have inferior quality.
Although golf ball technology is a significant user of thermoplastic polyurethane, scuff-resistant, durable materials also are important in other technologies. For example, articles such as livestock identity tags, shoe soles, luggage tags, and sports balls of many types, require a durable, tough product, good Shore D hardness, and flexural modulus. A scuff-resistant surface also may be useful.
Accordingly, for the foregoing reasons, there is a need in the art for a system and method that addresses the shortcomings of the prior art discussed above. In particular, there exists a need for a thermoplastic polyurethane elastomer that is durable and resistant to scuffing from use.